think of energy (MWh) as the equivalent of distance (miles or kilometres), and power (MW) as the equivalent of speed (mph or kph)

Electricity (confusingly also normally called power) and gas trades are often described in terms of a rate of energy, e.g. Megawatts, or therms per hour

However energy trades are priced in terms of energy(e.g. €45.3/MWh) so we need to be able to calculate the number of MWh of the trade or delivery period

you can't meaningfully add two values in Megawatts at different times - what does it mean to add two speeds together at different points on the Motorway?

If I drive 60 mph for 10 minutes, then 72 mph for the next 5 minutes, does the number 132 mph mean anything? (No!)

If I flow 10 MW one day and 20 MW the next day, the value 30 MW has no meaning

you can meaningfully add two values in Megawatt hours at different times

If I drive 10 miles in the first ten minutes, then 6 miles in the next five minutes, then I have driven 16 miles in total

If I flow 240 MWh one day and 480 MWh the next day, then I have flowed 720 MWh over the two days

you can't normally price something in Megawatts - a toll road makes you pay per mile, it doesn't matter how fast you went